Hydroxytyrosol acitretin ester with antioxidant activity and a method of preparing the same

ABSTRACT

A compound having the formula (I): 
                         
is disclosed. A method of preparing the compound of formula (I) is also disclosed.

FIELD OF THE INVENTION

The present invention relates to a food chemistry area, more specifically, a hydroxytyrosol acitretin ester with antioxidant activity and a method of preparing the same.

BACKGROUND OF THE INVENTION

Oil oxidation is one of the main reasons for the oxidative rancidity of oily food. The oxidation reaction of fats and oils includes three types: auto-oxidation, photosensitive oxidation and enzymatic oxidation. Auto-oxidation is the most common oxidation reaction in the process of oil storage. It is mainly the free radical reaction between unsaturated fatty acids and ground state oxygen, including chain initiation, chain transfer and chain termination. The oxidation of fats and oils will produce hydroperoxides, which are unstable and easily decompose to form aldehydes, ketones, acids and other substances. These substances not only affect the flavor and color of food and reduce the quality of food, but also produce many toxic substances, which endanger human health.

Hydroxytyrosol (compound of formula (II)) is a phenolic substance mainly found in olives and its processed products. It has a variety of physiological functions such as strong anti-oxidation, cancer prevention, antibacterial, anti-inflammatory, and lipid-lowering. It has great potential in the development and utilization of cosmetics.

Acitretin (compound of formula (III)) is a synthetic analogue of aromatic vitamin A, which is the active metabolite of etretinate, and is used to treat severe psoriasis, keratinization diseases and other skin diseases.

In the present invention, hydroxytyrosol is modified by acitretin to obtain a hydroxytyrosol acitretin ester. Antioxidant experimental data show that this ester has excellent antioxidant activity and has high medical research and application value in the field of antioxidant health products.

SUMMARY OF THE INVENTION

In one embodiment, the present invention provides a hydroxytyrosol acitretin ester that has the following Formula I:

In another embodiment, the present invention provides a method of preparing the compound of formula (I). The method includes reacting the compound of formula (II) with the compound of formula (III) in organic solvent to obtain the compound of formula (I):

In another embodiment, the reaction of the compound of formula (II) with the compound of formula (III) includes the following steps: placing the compound of formula (II) and the compound of formula (III), in a molar ratio of 1:1 to 1:1.3, in a reactor; adding an organic solvent and a catalytic amount of EDC to obtain a reaction mixture; and heating the reaction mixture at 60-90° C. for 4-8 hours; and purifying the crude product on a silica gel fresh chromatography column with petroleum ether and ethyl acetate as an eluent to obtain the compound of formula (I).

In another embodiment, the organic solvent is toluene, tetrahydrofuran or acetonitrile.

In another embodiment, the organic solvent is toluene.

In another embodiment, the molar ratio of the compound of formula (II) and the compound of formula (III) is 1:1.1.

In another embodiment, the reaction mixture is heated at 80° C.

In another embodiment, the reaction mixture is heated for 6 hours.

In another embodiment, the eluent is petroleum ether:ethyl acetate=1:3.

In another embodiment, the reaction of the compound of formula (II) with the compound of formula (III) includes the following steps: placing the compound of formula (II), a catalyst, and an ionic liquid in a reactor under nitrogen atmosphere, the catalyst being 12-molybdosilicic acid hydrate (H₆Mo₁₂O₄₁Si); adding the compound of formula (III) to the reactor to form a reaction mixture; heating the reaction mixture at 30-60° C. for 6-12 hours; placing the reaction mixture in a separating funnel to separate a crude product; purifying the crude product by recrystallization in methanol to obtain the compound of formula (I); and recycling the ionic liquid.

In another embodiment, the ionic liquid is 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]).

In another embodiment, the molar ratio of the compound of formula (II) and the compound of formula (III) is 1:1.1.

In another embodiment, the reaction mixture is heated at 40° C.

In another embodiment, the reaction mixture is heated for 8 hours.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.

In the drawings:

FIG. 1 shows the scavenging rate of the hydroxytyrosol acitretin ester (compound of formula (I)) and control solutions on DPPH free radicals.

FIG. 2 shows the scavenging rate of the hydroxytyrosol acitretin ester and the control solutions on hydroxyl radicals.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Reference will now be made in detail to embodiments of the present invention, example of which is illustrated in the accompanying drawings. The following examples illustrate the present invention, but the present invention is not limited to the following examples.

Example 1 Preparation of Compound of Formula (I): (2E,4E,6E,8E)-3,4-dihydroxyphenethyl 9-(4-methoxy-2,3,6-trimethylphenyl)-3,7-dimethylnona-2,4,6,8-tetraenoate

In a 250 mL three-necked flask, 77.0 mg (0.50 mmol) of hydroxytyrosol and 95.7 mg (0.50 mmol) EDC (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide) were dissolved in 60 mL of toluene under nitrogen atmosphere. 179.4 mg (0.55 mmol) of acitretin was dissolved in 60 mL of toluene, and added dropwise to the reaction mixture by a separatory funnel. After the completion of the dropwise addition, the temperature was raised to 80° C., and the reaction was carried out for 6 hours. Thin layer chromatography was used to track the reaction to completion, heating was stopped, and the protective device was removed. The concentrated solution was washed in water, extracted with ethyl acetate, dried and concentrated to obtain a crude product. The crude product was further purified by silica gel column chromatography eluting with petroleum ether:ethyl acetate=1:3 as eluent, and the eluent was concentrated under reduced pressure and dried to 179.9 mg of the title compound, a yield of 77.85%.

¹H-NMIR (400 MHz, DMSO-d₆) δ (ppm): 8.53 (2H, s), 7.16 (1H, s), 6.72 (3H, s), 6.46 (3H, s), 6.33 (2H, s), 5.82 (1H, s), 3.79 (2H, t), 3.53 (3H, s), 3.32 (2H, t), 2.59 (3H, s), 2.32 (9H, s), 2.11 (3H, s); ¹³C-NMR (400 MHz, DMSO-d₆) δ (ppm): 168.2, 156.1, 151.9, 139.2, 135.6, 130.8, 128.5, 119.8, 115.8, 110.6, 63.0, 55.8, 40.6, 21.5, 17.6, 13.9, 13.1, 12.1.

Example 2 Preparation of Compound of Formula (I)

In a 250 mL three-necked flask, 77.0 mg (0.50 mmol) of hydroxytyrosol and 95.7 mg (0.50 mmol) EDC were dissolved in 60 mL of acetonitrile under nitrogen atmosphere. 179.4 mg (0.55 mmol) of acitretin was dissolved in 60 mL of acetonitrile, and added dropwise to the reaction mixture by a separatory funnel. After the completion of the dropwise addition, the temperature was raised to 90° C., and the reaction was carried out for 4 hours. Thin layer chromatography was used to track the reaction to completion, heating was stopped, and the protective device was removed. The concentrated solution was washed in water, extracted with ethyl acetate, dried and concentrated to obtain a crude product. The crude product was further purified by silica gel column chromatography eluting with petroleum ether:ethyl acetate=1:3 as eluent, and the eluent was concentrated under reduced pressure and dried to 166.8 mg of the title compound, a yield of 72.16%.

Example 3 Preparation of Compound of Formula (I)

In a 250 mL three-necked flask, 77.0 mg (0.50 mmol) of hydroxytyrosol and 95.7 mg (0.50 mmol) EDC were dissolved in 60 mL of tetrahydrofuran under nitrogen atmosphere. 195.7 mg (0.60 mmol) of acitretin was dissolved in 60 mL of tetrahydrofuran, and added dropwise to the reaction mixture by a separatory funnel. After the completion of the dropwise addition, the temperature was raised to 80° C., and the reaction was carried out for 7 hours. Thin layer chromatography was used to track the reaction to completion, heating was stopped, and the protective device was removed. The concentrated solution was washed in water, extracted with ethyl acetate, dried and concentrated to obtain a crude product. The crude product was further purified by silica gel column chromatography eluting with petroleum ether:ethyl acetate=1:3 as eluent, and the eluent was concentrated under reduced pressure and dried to 157.0 mg of the title compound, a yield of 67.96%.

Example 4 Preparation of Compound of Formula (I)

In a 250 mL three-necked flask, 77.0 mg (0.50 mmol) of hydroxytyrosol and 95.7 mg (0.50 mmol) EDC were dissolved in 60 mL of toluene under nitrogen atmosphere. 179.4 mg (0.55 mmol) of acitretin was dissolved in 60 mL of toluene, and added dropwise to the reaction mixture by a separatory funnel. After the completion of the dropwise addition, the temperature was raised to 75° C., and the reaction was carried out for 6 hours. Thin layer chromatography was used to track the reaction to completion, heating was stopped, and the protective device was removed. The concentrated solution was washed in water, extracted with ethyl acetate, dried and concentrated to obtain a crude product. The crude product was further purified by silica gel column chromatography eluting with petroleum ether:ethyl acetate=1:3 as eluent, and the eluent was concentrated under reduced pressure and dried to 157.9 mg of the title compound, a yield of 68.32%.

Example 5 Preparation of Compound of Formula (I)

In a 250 mL three-necked flask, 77.0 mg (0.50 mmol) of hydroxytyrosol and 95.7 mg (0.50 mmol) EDC were dissolved in 60 mL of acetonitrile under nitrogen atmosphere. 179.4 mg (0.55 mmol) of acitretin was dissolved in 60 mL of acetonitrile, and added dropwise to the reaction mixture by a separatory funnel. After the completion of the dropwise addition, the temperature was raised to 60° C., and the reaction was carried out for 8 hours. Thin layer chromatography was used to track the reaction to completion, heating was stopped, and the protective device was removed. The concentrated solution was washed in water, extracted with ethyl acetate, dried and concentrated to obtain a crude product. The crude product was further purified by silica gel column chromatography eluting with petroleum ether:ethyl acetate=1:3 as eluent, and the eluent was concentrated under reduced pressure and dried to 153.9 mg of the title compound, a yield of 66.57%.

Example 6 Preparation of Compound of Formula (I)

In a 250 mL three-necked flask, 77.0 mg (0.50 mmol) of hydroxytyrosol and 95.7 mg (0.50 mmol) EDC were dissolved in 60 mL of tetrahydrofuran under nitrogen atmosphere. 179.4 mg (0.55 mmol) of acitretin was dissolved in 60 mL of tetrahydrofuran, and added dropwise to the reaction mixture by a separatory funnel. After the completion of the dropwise addition, the temperature was raised to 75° C., and the reaction was carried out for 5 hours. Thin layer chromatography was used to track the reaction to completion, heating was stopped, and the protective device was removed. The concentrated solution was washed in water, extracted with ethyl acetate, dried and concentrated to obtain a crude product. The crude product was further purified by silica gel column chromatography eluting with petroleum ether:ethyl acetate=1:3 as eluent, and the eluent was concentrated under reduced pressure and dried to 149.6 mg of the title compound, a yield of 64.72%.

Example 7 Preparation of Compound of Formula (I)

In a 250 mL three-necked flask, 77.0 mg (0.50 mmol) of hydroxytyrosol, 179.4 mg (0.55 mmol) of acitretin and 8.3 mg (0.005 mmol) silicomolybdic acid were dissolved in 100 mL of 1-butyl-3-methylimidazolium tetrafluoroborate under nitrogen atmosphere. After the reactants were fully dissolved, the temperature was raised to 40° C. and the reaction was carried out for 8 hours. Thin layer chromatography was used to track the reaction to completion, heating was stopped, and the protective device was removed. The reaction mixture system was allowed to separate into layers to give a crude product. The crude product was recrystallized with 80 mL methanol, and filtered and dried to obtain 198.3 mg of the title compound, a yield of 85.83%. 1-Butyl-3-methylimidazolium tetrafluoroborate was recovered for reuse.

Example 8 Preparation of Compound of Formula (I)

In a 250 mL three-necked flask, 77.0 mg (0.50 mmol) of hydroxytyrosol, 179.4 mg (0.55 mmol) of acitretin and 8.3 mg (0.005 mmol) silicomolybdic acid were dissolved in 100 mL of 1-butyl-3-methylimidazolium tetrafluoroborate under nitrogen atmosphere. After the reactants were fully dissolved, the temperature was raised to 60° C. and the reaction was carried out for 12 hours. Thin layer chromatography was used to track the reaction to completion, heating was stopped, and the protective device was removed. The reaction mixture system was allowed to separate into layers to give a crude product. The crude product was recrystallized with 50 mL methanol, and filtered and dried to obtain 195.8 mg of the title compound, a yield of 84.73%. 1-Butyl-3-methylimidazolium tetrafluoroborate was recovered for reuse.

Example 9

The Antioxidant Activity of the Hydroxytyrosol Acitretin Ester Measured by a DPPH Radical Scavenging Activity Assay

2,2-Diphenyl-1-picryl hydrazyl (DPPH) is an organic compound composed of a stable organic radical. In the DPPH molecule, due to the presence of multiple electron-withdrawing —NO2 and large π bonds of the benzene ring, nitrogen free radical is stabilized. Its methanol solution is purple and has a maximum absorption peak at 517 nm. After the addition of an antioxidant, DPPH captures an electron to be paired with the free electron, and the purple fades and turns into a yellow substance. The absorption at 517 nm disappears, and the degree of fading is quantitatively related to the number of electrons it captures. Based on this principle, a spectrophotometer is used to detect the change of the absorbance of the DPPH radical and the sample solution, and the ability of the sample to provide hydrogen atoms and scavenge free radicals can be measured.

Preparation of DPPH Solution:

Measuring exact amount of 2,2-diphenyl-1-picryl hydrazyl (DPPH) and dissolving in toluene to prepare a 0.2 mmoL/L DPPH solution; being stored at 0° C. in dark.

Preparation of Test Solutions:

Vc (vitamin C, positive control), hydroxytyrosol acitretin ester (sample), hydroxytyrosol (control) and acitretin (control). The test solutions were subjected to gradient dilution with toluene, and three sets of controls were separately dissolved in a test tube with a certain amount of toluene to prepare the same concentration gradient as the sample. The corresponding three groups of control solutions were obtained (gradient settings are shown in Table 1).

TABLE 1 Dilution gradient of the test solution Number Test solution Concentration gradient/ppm Vc Vc 1.76, 8.80, 21.12, 42.24, 79.20, 112.64, 281.60, 492.80, 792.00, 915.20 A Hydroxytyrosol 1.76, 8.80, 21.12, 42.24, 79.20, 112.64, acitretin ester 281.60, 492.80, 792.00, 915.20 B Hydroxytyrosol 1.76, 8.80, 21.12, 42.24, 79.20, 112.64, 281.60, 492.80, 792.00, 915.20 C Acitretin 1.76, 8.80, 21.12, 42.24, 79.20, 112.64, 281.60, 492.80, 792.00, 915.20

Measurements:

Solution absorbance measurement: taking 2 mL of sample solution (Table 1: Vc, B, C), adding 2 mL of DPPH solution with concentration of 2×10⁻⁴ moL/L, mixing and reacting in the dark at room temperature for 30 min, adjusting to zero with toluene, and measuring at 517 nm. The absorbance Ai was simultaneously measured for the absorbance Aj of 2 mL of toluene mixed with 2 mL of the sample solution and the absorbance A₀ of 2 mL of DPPH solution mixed with 2 mL of toluene. The experimental results are shown in Table 2.

TABLE 2 Absorbance test results of each test solution Concentration/ppm Sample Absorbance 1.76 8.80 21.12 42.24 79.20 112.64 281.60 492.80 792.00 915.20 Vc Ai 0.718 0.624 0.222 0.142 0.091 0.078 0.076 0.070 0.074 0.065 Aj 0.068 0.061 0.050 0.054 0.069 0.057 0.062 0.062 0.066 0.059 Ao 0.846 A Ai 0.822 0.787 0.649 0.543 0.327 0.233 0.190 0.145 0.104 0.086 Aj 0.048 0.049 0.055 0.046 0.059 0.051 0.058 0.047 0.055 0.047 Ao 0.895 B Ai 0.918 0.904 0.810 0.739 0.630 0.580 0.403 0.365 0.268 0.254 Aj 0.053 0.046 0.047 0.039 0.060 0.055 0.041 0.046 0.035 0.037 Ao 0.935 C Ai 0.851 0.830 0.788 0.737 0.631 0.562 0.423 0.384 0.281 0.266 Aj 0.051 0.045 0.049 0.043 0.050 0.047 0.046 0.041 0.035 0.039 Ao 0.862

Clearance rate is calculated in accordance with the following equation: Clearance rate (%)=[1−(Ai−Aj)/A ₀]*100%

Calculated clearance rate is shown in FIG. 1 and Table 3.

TABLE 3 DPPH clearance rate experiment results Clearance rate/% (n = 3) Concentration/ppm Vc A B C 1.76 23.16 13.54 7.42 7.21 8.80 33.47 17.53 8.16 8.90 21.12 79.63 33.65 18.43 14.23 42.24 89.55 44.52 25.10 19.53 79.20 97.42 70.11 38.99 32.55 112.64 97.53 79.66 43.87 40.23 281.60 98.29 85.21 61.25 56.21 492.80 99.06 89.03 65.88 60.25 792.00 99.10 94.55 75.03 71.42 915.20 99.28 95.62 76.76 73.65

Example 10

The Antioxidant Activity of the Hydroxytyrosol Acitretin Ester Measured by Hydroxyl Radical Scavenging Experiment

Hydroxyl radical scavenging experiments use Fenton reaction to generate hydroxyl radicals, and salicylic acid reacts with the generated hydroxyl radicals to generate 2,3-dihydroxybenzoic acid. This substance has absorption at 510 nm. When antioxidants are added to the reaction system, it can reduce the production of hydroxyl radicals, thereby reducing the production of 2,3-dihydroxybenzoic acid. The color development of H₂O₂ is quantitatively related to the total antioxidant capacity of antioxidants. By using a fixed time reaction method, the absorbance of the reaction solution containing the tested substance was measured at 510 nm and compared with the blank solution, so as to determine the scavenging effect of the tested substance on hydroxyl radical.

Reagent Preparation:

1.5 mmol/L FeSO₄: 0.0417 g FeSO₄ in a 100 mL volumetric flask, adjusting the volume to the scale with distilled water; 6 mmol/L H₂O₂: 61 μL 30% H₂O₂ in a 100 mL volumetric flask, adjusting the volume to the scale with distilled water; 20 mmol/L salicylic acid: 0.2765 g salicylic acid in a 100 mL volumetric flask, adjusting the volume to the scale with ethanol.

Preparation of Sample Solution:

The hydroxytyrosol acitretin ester was diluted with acetonitrile according to the concentration gradient of 0.04, 0.07, 0.21, 0.35, 1.06, 1.76, 5.28, 8.80 (mg/mL) to prepare the sample solution to be tested. In the same way, the control samples (vitamin C, hydroxytyrosol, acitretin) were diluted with the same concentration gradient with methanol to obtain the corresponding 3 groups of reference solution solutions. Gradient settings are shown in Table 4.

TABLE 4 Dilution gradient of the test solution Number Test solution Concentration gradient/(mg/mL) Vc Vc 0.04, 0.07, 0.21, 0.35, 1.06, 1.76, 5.28, 8.80 A Hydroxytyrosol 0.04, 0.07, 0.21, 0.35, 1.06, 1.76, 5.28, 8.80 acitretin ester B Hydroxytyrosol 0.04, 0.07, 0.21, 0.35, 1.06, 1.76, 5.28, 8.80 C Acitretin 0.04, 0.07, 0.21, 0.35, 1.06, 1.76, 5.28, 8.80

Measurements:

Taking 1 mL of the sample to be tested, add ign2 mL FeSO₄ solution with a concentration of 1.5 mmol/L and 0.6 mL salicylic acid with a concentration of 20 mmol/L, and finally adding 1.4 mL of H₂O₂ solution with a concentration of 6 mmol/L; reacting in a water bath for 30 minutes, and adjusting to zero with absolute ethanol; measuring the absorbance A_(i) at 510 nm, measuring the absorbance A_(j) of 1.4 mL distilled water instead of H₂O₂ and the absorbance A₀ of 1 mL distilled water instead of the sample. The experimental results are shown in Table 5.

TABLE 5 Absorbance test results of each test solution Concentration/ppm Sample Absorbance 0.04 0.07 0.21 0.35 1.06 1.76 5.28 8.80 Vc Ai 0.822 0.723 0.450 0.183 0.115 0.132 0.134 0.129 Aj 0.041 0.047 0.051 0.065 0.079 0.101 0.115 0.120 Ao 1.125 A Ai 0.943 0.840 0.738 0.691 0.653 0.575 0.528 0.446 Aj 0.038 0.052 0.063 0.075 0.081 0.091 0.111 0.128 Ao 1.221 B Ai 1.036 1.003 0.998 0.964 0.912 0.857 0.817 0.642 Aj 0.031 0.032 0.055 0.071 0.084 0.097 0.152 0.178 Ao 1.054 C Ai 1.053 1.043 0.992 0.946 0.892 0.854 0.776 0.636 Aj 0.028 0.033 0.037 0.041 0.053 0.066 0.079 0.105 Ao 1.059

Clearance rate is calculated in accordance with the following equation: Clearance rate (%)=[1−(Ai−Aj)/A ₀]*100%

Calculated clearance rate is shown in FIG. 2 and Table 6.

TABLE 6 DPPH clearance rate experiment results Clearance rate/% (n = 3) Concentration/ppm Vc A B C 0.04 30.55 25.86 4.63 3.21 0.07 39.85 35.42 7.86 4.65 0.21 64.51 44.69 10.52 9.78 0.35 89.55 49.51 15.27 14.55 1.06 96.77 53.17 21.44 20.74 1.76 97.21 60.35 27.85 25.63 5.28 98.35 65.86 36.95 34.19 8.80 99.18 73.95 55.98 49.86

As shown in Figures. 1-2 and Tables 1-6, the antioxidant activity of the hydroxytyrosol acitretin ester (A) was concentration-dependent, and the scavenging activity of the hydroxytyrosol acitretin ester to DPPH radical and hydroxyl radical increased with the increase of concentration. In the concentration range, the highest scavenging rate of DPPH free radical was 95.62%, and the highest scavenging rate of hydroxyl radical was 73.95%. The hydroxytyrosol acitretin ester has similar scavenging ability to the positive control Vc group. Compared with the control group treated only with hydroxytyrosol (B) and acitretin (C), the hydroxytyrosol acitretin ester had the strongest scavenging ability to free radicals at the same concentration. The antioxidant activity at high concentration was much higher than that of hydroxytyrosol (B) control group and acitretin (C) control group. The experimental results show that the compound has excellent antioxidant activity and has a good application prospect. 

What is claimed is:
 1. A compound having the following formula (I):


2. A method of preparing the compound of formula (I) of claim 1, comprising: reacting a compound of formula (II) with a compound of formula (III) to obtain the compound of formula (I):


3. The method of claim 2, wherein the reaction of the compound of formula (II) with the compound of formula (III) comprises the following steps: placing the compound of formula (II) and the compound of formula (III), in a molar ratio of 1:1 to 1:1.3, in a reactor; adding an organic solvent and a catalytic amount of EDC under nitrogen atmosphere to obtain a reaction mixture; and heating the reaction mixture at 60-90° C. for 4-8 hours; and purifying the crude product on a silica gel fresh chromatography column with petroleum ether and ethyl acetate as an eluent to obtain the compound of formula (I).
 4. The method of claim 3, wherein the organic solvent is toluene, tetrahydrofuran, or acetonitrile.
 5. The method of claim 4, wherein the organic solvent is toluene.
 6. The method of claim 3, wherein the molar ratio of the compound of formula (II) and the compound of formula (III) is 1:1.1.
 7. The method of claim 3, wherein the reaction mixture is heated at 80° C.
 8. The method of claim 3, wherein the reaction mixture is heated for 6 hours.
 9. The method of claim 3, wherein the eluent is petroleum ether:ethyl acetate=1:3.
 10. The method of claim 2, wherein the reaction of the compound of formula (II) with the compound of formula (III) comprises the following steps: placing the compound of formula (II), a catalyst, and an ionic liquid in a reactor under nitrogen atmosphere, the catalyst being 12-molybdosilicic acid hydrate (H₆Mo₁₂O₄₁Si); adding the compound of formula (III) to the reactor to form a reaction mixture; heating the reaction mixture at 30-60° C. for 6-12 hours; placing the reaction mixture in a separating funnel to separate a crude product; purifying the crude product by recrystallization in methanol to obtain the compound of formula (I); and recycling the ionic liquid.
 11. The method of claim 10, wherein the ionic liquid is 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]).
 12. The method of claim 10, wherein the compound of formula (II) and the compound (III) have a molar ratio of 1:1 to 1:1.3.
 13. The method of claim 12, wherein the molar ratio of the compound of formula (II) and the compound of formula (III) is 1:1.1.
 14. The method of claim 10, wherein the reaction mixture is heated at 40° C.
 15. The method of claim 10, wherein the reaction mixture is heated for 8 hours. 